Readme implies you can send empty G2/G3 commands, but this causes a shutdown of marlin. Also clarify expected M115 response.

[Dweinbach]

I have a machine with Marlin 2.1.3 with arc enabled.

After installing Arcwelder, I tried to follow the readme and typed "G2" (and later "G3") in terminal, and both casued a full emergency shutdown of the printer requiring a power cycle.

the readme says: "For earlier versions you can send an empty G2 or G3 command". I tried this only because M115 did not return the exact string "ARC_SUPPORT" anywhere.

I realize these seem picky, but to a person just trying to follow directions, these issues in readme.md tripped me up. here is my proposed edit:

For recent versions of Marlin (2.0.6 and above), you can send an M115 and look for "Cap:ARCS:1" to see if your firmware has ARC_SUPPORT enabled. For earlier versions you can send a G2 or G3 command (try "G2 I20 J20"). If your printer responds with unknown command, arc support is not enabled.